1. Field of the Invention
The present invention generally relates to a method for synthesizing zeolite beta, and more particularly to a method for synthesizing all-silica zeolite beta with small crystal size.
2. The Prior Arts
Zeolite beta is synthesized by hydrothermal treatment first described in 1967 in U.S. Pat. No. 3,308,069. Zeolite beta has a three-dimensional large-pore system of a 12-membered ring opening 0.76 nm wide, and draws much attention because of its relative high thermal stability, unique characteristics, in particular its acidity and potential for acid catalysis. According to literature, zeolite beta possesses high activity in cracking, isomerization, cyclization, alkylation, and hydrocracking of the alkanes, and plays an important role in the petroleum industry.
When zeolite beta with high surface area and large pore volume is used as a heterogeneous catalyst, it can exhibit high reactivity and selectivity in the acid catalyzed reactions. The heterogeneous catalytic reaction involves adsorption, diffusion, chemical reaction and desorption steps. The heterogeneous catalytic reaction occurs at or very near the fluid-solid interface, and thereby catalytic performance can be improved by increasing geometric surface area. The smaller the particle size is, the higher the surface area is. When the surface area of the catalyst is higher, it has more catalytic active sites, and thereby the catalytic performance becomes better. Also, research has been focused recently on the development of new methods for preparation of highly silicious zeolites exhibiting extremely low acidity, such as high-silica zeolites, and all-silica zeolites. Therefore, it is desirable to find an economic and efficient way to reach these goals.
Zeolite beta was synthesized by hydrolyzing an aqueous solution of a synthetic mixture comprising a silica source (such as tetraalkyl orthosilicate, Si(OR)4) and an aluminium source in the presence of a templating agent, nucleating the resulting product under stirring at room temperature (15 to 30° C.), followed by crystallization at higher temperatures and pressures, and finally drying the resulting product. In the above process, the hydrolysis product of Si(OR)4 with water contains a certain amount of silanol Si—OH groups, and these silanol Si—OH groups tend to condense by creating Si—O—Si bonds. When Si(OR)4 was hydrolyzed at room temperature, the hydrolysis process took very long time to complete. Since the hydrolysis went on exceedingly slowly, an acid or alkaline catalyst (such as tetraethylammonium hydroxide) was usually added to accelerate the hydrolysis process.
Reference is made to U.S. Pat. No. 5,310,534, wherein the synthesis method of highly silicious zeolite beta with silica-to-alumina ratio of more than 4000 was described, and in which dealuminization was achieved by acid treatment. The drawbacks are that the highly silicious zeolite beta was obtained with only 80% crystallinity due to the presence of considerable defects in its crystalline structure. Reference is also made to U.S. Pat. No. 5,554,356, wherein all-silica zeolite beta was synthesized using 4,4′-trimethylenebis(N-benzylpiperidine) as an organic template. The drawbacks are that this synthesis method is not easily commercialized because 4,4′-trimethylenebis(N-benzylpiperidine) is not commercially cheap available. Reference is made to WO97/33830 and D. P. Serrano et al. in “Crystallization mechanism of all-silica zeolite beta in fluoride medium”, published in Microporous and Mesoporous Materials, 46, pp. 35-46, 2001, wherein all-silica zeolite beta was synthesized using tetraethyl orthosilicate as a source of silicon dioxide, and tetraethylammonium hydroxide as an organic template, in the presence of fluoride ions. The drawbacks are that all-silica zeolite beta with a broad crystal size distribution is obtained, and most of its crystal sizes are larger than 10 μm. Also, the synthesis method disclosed by WO97/33830 and D. P. Serrano et al. only suitably applied in small-scale production. If all-silica zeolite beta is produced in large scale quantities by the method disclosed by WO97/33830 and D. P. Serrano et al., the crystallinity of all-silica zeolite beta obtained will become non-uniform and become very lower than the original product. However, when the all-silica zeolite beta is prepared according to the same methods as disclosed by WO97/33830 and D. P. Serrano et al except for using the source of silicon dioxide other than tetraethyl orthosilicate, it shows that the aging process takes about seven or more days for growing high quality crystals.
Reference is made to Camblor et al, Chem. Commun., 2365, 1996, wherein zeolite beta could be successfully synthesized via the fluoride route in absence of aluminum or titanium. Camblor et al. found that fluoride ions reside within the small cage located around the periphery of the central pore space of zeolite beta, showing some “templating” role for the formation of zeolite beta crystals. However, in the method disclosed by Camblor et al., the aging process was carried out at room temperature, and thereby there may be a problem with ethanol left in the hydrolysis product, which will hinder the crystallization of all-silica zeolite beta. Consequently, the overall synthesis time becomes long for growing high quality crystals when using the method disclosed by Camblor et al.
Reference is made to Spain Pat. No. P9501552, wherein zeolite beta with a high silica-to-alumina mole ratio was synthesized using Al-containing zeolite beta as seed. The overall synthesis time was shortened due to the presence of Al2O3.
In the prior art, zeolite beta was typically synthesized using tetraethyl orthosilicate as a source of silicon dioxide, which was easily hydrolized. However, the disadvantages for using tetraethyl orthosilicate are that tetraethyl orthosilicate is toxic, its purchase price is high, the evaporation of ethanol is incomplete after aging, and zeolite beta product is obtained in low yield. However, although a method for synthesizing zeolite beta using colloidal silica or fumed silica has been reported in several literatures, the overall synthesis time reported in these literatures usually was 15 or more days, and also zeolite beta with large crystal size was obtained.
As stated above, zeolite beta with small crystal size is the most desired catalyst for heterogeneous catalytic reactions due to its increased active acidic sites and increased three dimensional interface with the support and reactant. Therefore, for most industrial purposes, the demand is that zeolite beta has a smaller and more uniform crystal size.
According to the above, in order to solve the drawbacks of prior art, the present invention gives a fast and efficient way of synthesis of all-silica zeolite beta with small crystal size.